White dwarfs can be classified into several spectral types in terms
of the spectral characteristics. The current classification system
was introduced by Sion et al. ([1]}) and has been modified
several times. In this system, the spectral type of a white dwarf is
denoted by a letter D plus another letter indicating its spectral
characteristics. Sometimes, a suffix is added to indicate some other
features (polarization, magnetic field, pulse, etc.). Table
REF  lists a spectral classification scheme from McCook &
Sion ([2]}).
<TABLE>